The amount of cross-sectional area narrowing by atherosclerotic plaques was determined histologically in each 5-mm long segment of the entire lengths of the right, left main, left anterior descending and left circumflex coronary arteries in 40 patients with fatal coronary heart disease and known fasting serum total cholesterol (TC) and triglyceride (Tg) levels. The pathents were divided into four groups based upon the TC and Tg levels: group I = TC less than or equal to 250 mg/dl, Tg less than or equal to 170 mg/dl; group II = TC less than or equal to 250, Tg more than 170; group III = TC more than 250, Tg less than or equal to 170 and group IV = TC more than 250, Tg more than 170. The number of 5-mm segments of coronary artery narrowed severely (76-100% in cross-sectional area) by atherosclerotic plaques in each group was as follows: 172 (34%) of 505 five mm segments from group I; 242 (69%) of 353 segments from group II; 120 (41%) of 295 from group III and 425 (48%) of 884 segments from group IV. The mean percent of 5-mm segments narrowed severely was significantly greater in group II than in group I (p less than .005) or group III (p less than .01). Additionally, the mean number of 4 coronary arteries per patient severely narrowed and the number of pathents with severe narrowing of the left main coronary artery were significantly greater in groups II and III than in group I. The percents of 5-mm segments narrowed severely correlated significantly with the serum Tg level (p less than .03).